COORDINATION. 619 



under the thumb-nail, more than a mere withdrawal of the hand 

 takes place. The entire arm is violently shaken with the ob- 

 vious purpose of shaking off the offending stimulus before the will 

 has time to come into operation. Here, then, we have a complex 

 form of coordination of purposeful muscular movement, as the 

 immediate result of an impulse coming from the skin, and this 

 coordination is the result of mutual relationships existing be- 

 tween the cells of the cord employed in transmitting both sen- 

 sory and motor impulses. Not only is the movement a regular 

 and coordinated act, but in many cases, as has just been men- 

 tioned, it is performed with a definite purpose, as if it were the 

 result of thought, but since there need be no consciousness it 

 cannot be mental. All these points may be easily studied on a 

 frog killed about an hour beforehand by decapitation. If a frag- 

 ment of blotting-paper, moistened in weak acid, be placed on the 

 skin of the body of such an animal, in a position not easily 

 reached by the foot, a most complex series of movements follows, 

 first with one leg then the other, or with both. The muscular 

 action is both elaborately coordinated and purposeful ; indeed, 

 the movements of the headless animal might be called ingenious, 

 and usually result in the removal of the offending paper. 



If the degree of the stimulation be carefully regulated, it will be 

 found that the results obtained by peripheral stimulation depend 

 on (a) the strength of the stimulus, and the length of time for 

 which it is applied ; (6) the degree of excitability of the cells of 

 the cord ; (c) the readiness with which the impulses pass along 

 the thin conducting channels to the gray matter, and (d) the 

 functional activity of the muscles which act as the indicators of 

 the reflex effects. 



By graduating the strength of the solution of acid with which 

 a square millimetre of blotting-paper is saturated before it is 

 placed on a frog's foot, the following results are obtained : When 

 very weak acid is employed, only slight local and unilateral 

 movement is caused. If steeped in stronger acid, the same sized 

 paper produces a series of reflex movements, spreading to several 

 muscles on both sides of the body. If the stimulus be further 

 strengthened, the movements become violent and more extended 





